Invasive strains of Shigella and enteroinvasive E. coli have been shown to carry a plasmid (the invasiveness plasmid) that encodes genes necessary for virulence. (Hale, et al., Infect. Immun. 40: 340-350 (1983)) Based on identification of critical regions on this plasmid, DNA probes have been developed that identify potentially invasive isolates (Boileau, et al., J. Clin. Microbiol. 20:(1984) Small and Falkow (In L. Leive (ed.), Microbiology-1986. American Society for Microbiology, Washington, D.C., pp. 121-124) had described a 2.5 kilobase HindIII fragment of the invasiveness plasmid from an enteroinvasive E. coli strain for use in testing invasivness of the enteric organisms. Previously, it had been demonstrated that virulence of these organisms could be predicted by inoculating the conjunctival sac of a guinea pig with a strain of Shigella. Invasive strains produced a purulent keratoconjunctivitis associated with invasion of corneal epitheial cells and infiltration with polymorphonuclear leukocytes similar to that seen in intestinal lesions caused by Shigellae.
The use of guinea pigs in the identification of pathogenic organisms in the food supply and in clinical speciments is clearly expensive and time-consuming. The use of large fragments such as the 17-kilobase EcoRI fragment of Boileau and the 2.5-kilobase HindIII fragment of Small and Falkow are expensive to make and use.